This application is filed under the provisions of 35 U. S.C. xc2xa7371 and claims the priority of International Patent Application No. PCT/DE01/00206 filed Jan. 17, 2001, which in turn claims priority of German Patent Application No. 100 09 471.6 filed Feb. 28, 2000.
1. Field of Technology
The invention relates to a method for the preparation of a solution with increased thermal stability of cellulose in aqueous amine oxide, preferably N-methylmorpholine-N-oxide, for the processing into cellulose molded articles according to the Lyocell method, where pulp is suspended in aqueous amine oxide and the suspension is converted into the cellulose solution.
2. Description or Related Art
From literature (K. Gxc3x6tze Chemiefasern nach dem Viskoseverfahren [Chemical fibers according to the viscose method], Volume I, page 362 to 369, published by Springer, Berlin/Heidelberg/New York, 1967) it is known that the reduction of cellulose strongly depends on the pH value and that in the presence of oxidation agents, the ratio of reducing carbonyl- and hydroxyl groups to carboxyl groups also strongly depends on the pH value.
The dissolution of cellulose in aqueous amine oxides in heat also leads to reduction processes because the amine oxides on the one hand represent oxidation agents with weakly basic properties, and on the other hand, the reduction easily leads to byproducts such as primary and secondary amines, which have strong alkaline properties.
DD 218 104 claims the addition of one or more substances with an alkaline effect in quantities of 0.1 to 10% relative to the cellulose solution for the stabilization of melts comprised of cellulose and amine oxides.
U.S. Pat No. 4 324 593 proposes an addition of amines, aqueous ammonia or alkaline hydroxides to increase the dissolution speed of cellulose in amine oxides. The acceleration effect supposedly depends on the change of the pH-value.
WO 95/23837 proposes for increasing the stability of the solutions of cellulose in amine oxides, preferably N-methylmorpholine-N-oxide, that the regenerated tertiary amine oxide has a pH-value depending on the amine oxide concentration A according to the following equation
pH=xe2x88x920.0015xc3x97A2+0.2816xc3x97A+f
and that said value is adjusted by adding alkaline substances and/or by passing the amine oxides over ion exchangers. The equation is supposed to apply in the range of 40% xe2x89xa6A xe2x89xa786% with f=1.00 for the upper limit and f=xe2x88x921.80 for the lower limit.
Unfortunately, increasing the stability by adjusting the pH-value of the amine oxide solution is described only for solutions that comprise both gallic acid propyl ester and hydroxylamine as stabilizers. The component of the stabilization that can be attributed solely to the adjustment of the pH-value is not mentioned.
From the definition of the pH-value as a negative logarithm of the hydrogen ion activity, it can be concluded that it applies only for very diluted aqueous solutions and changes strongly with the temperature due to the temperature dependence of the movements of the ions. The measurement of the pH-value with solutions of 86% NMMO having a melting point of approximately 70xc2x0 C. thus does not appear to make much sense.